Biometric authentication: what method works best?

There does not appear to be any one method of biometric data gathering and reading that does the "best" job of ensuring secure authentication. Each of the different methods of biometric identification have something to recommend them. Some are less invasive, some can be done without the knowledge of the subject, some are very difficult to fake.

Face recognition
Of the various biometric identification methods, face recognition is one of the most flexible, working even when the subject is unaware of being scanned. It also shows promise as a way to search through masses of people who spent only seconds in front of a "scanner" - that is, an ordinary digital camera.
Face recognition systems work by systematically analyzing specific features that are common to everyone's face - the distance between the eyes, width of the nose, position of cheekbones, jaw line, chin and so forth. These numerical quantities are then combined in a single code that uniquely identifies each person.

Fingerprint identification
Fingerprints remain constant throughout life. In over 140 years of fingerprint comparison worldwide, no two fingerprints have ever been found to be alike, not even those of identical twins. Good fingerprint scanners have been installed in PDAs like the iPaq Pocket PC; so scanner technology is also easy. Might not work in industrial applications since it requires clean hands.
Fingerprint identification involves comparing the pattern of ridges and furrows on the fingertips, as well as the minutiae points (ridge characteristics that occur when a ridge splits into two, or ends) of a specimen print with a database of prints on file.

Hand geometry biometrics
Hand geometry readers work in harsh environments, do not require clean conditions, and forms a very small dataset. It is not regarded as an intrusive kind of test. It is often the authentication method of choice in industrial environments.

Retina scan
There is no known way to replicate a retina. As far as anyone knows, the pattern of the blood vessels at the back of the eye is unique and stays the same for a lifetime. However, it requires about 15 seconds of careful concentration to take a good scan. Retina scan remains a standard in military and government installations.

Iris scan
Like a retina scan, an iris scan also provides unique biometric data that is very difficult to duplicate and remains the same for a lifetime. The scan is similarly difficult to make (may be difficult for children or the infirm). However, there are ways of encoding the iris scan biometric data in a way that it can be carried around securely in a "barcode" format. (See the SF in the News article Biometric Identification Finally Gets Started for some detailed information about how to perform an iris scan.)

Signature
A signature is another example of biometric data that is easy to gather and is not physically intrusive. Digitized signatures are sometimes used, but usually have insufficient resolution to ensure authentication.

Voice analysis
Like face recognition, voice biometrics provide a way to authenticate identity without the subject's knowledge. It is easier to fake (using a tape recording); it is not possible to fool an analyst by imitating another person's voice.